# Synapse A **synapse** is the functional junction between the axon terminal of one neuron (the presynaptic neuron) and the dendrite of another (the postsynaptic neuron). It is the site where information is relayed between neurons. ## Structure of a Synapse Neurons are not in direct physical contact. They are separated by a microscopic gap. The three main components of a synapse are: 1. **Presynaptic Neuron:** The neuron that carries an impulse *towards* the synapse. Its axon ends in a rounded structure called the **synaptic knob**. 2. **Postsynaptic Neuron:** The neuron that *receives* the impulse after it crosses the synapse. 3. **Synaptic Cleft:** The narrow gap (about 20-30 nm for a chemical synapse) that separates the presynaptic and postsynaptic neurons. The **synaptic knob** (axon terminal) is the distal end of the presynaptic axon, which contains **Synaptic Vesicles**: Membranous sacs filled with chemical messengers called **neurotransmitters**. ## Mechanism of Synaptic Transmission Synaptic transmission is the process of transmitting a nerve impulse across the synaptic cleft, primarily via chemical messengers. 1. **Arrival of Action Potential:** An action potential (nerve impulse) travels down the axon and arrives at the synaptic knob of the presynaptic neuron. 2. **Calcium Ion Influx:** The arrival of the action potential opens voltage-gated **calcium channels** in the presynaptic membrane. Since $Ca^{2+}$ concentration is higher outside the neuron, $Ca^{2+}$ ions rush into the synaptic knob. 3. **Vesicle Fusion and Neurotransmitter Release:** The influx of $Ca^{2+}$ causes synaptic vesicles to move towards and fuse with the presynaptic membrane. This process, called *exocytosis*, releases neurotransmitter molecules into the synaptic cleft. 4. **Diffusion Across Cleft:** The released neurotransmitter molecules diffuse across the narrow synaptic cleft. 5. **Binding to Receptors:** Neurotransmitters bind to specific receptor proteins on the membrane of the postsynaptic neuron. 6. **Postsynaptic Potential Generation:** This binding opens ion channels (e.g., $Na^+$ channels) in the postsynaptic membrane, causing it to depolarize. This change in voltage is called an **Excitatory Postsynaptic Potential (EPSP)** because it brings the neuron's membrane potential closer to the threshold required to fire a new action potential. 7. **Signal Termination:** To prevent continuous stimulation, the neurotransmitter is quickly removed from the synaptic cleft. This is typically done by enzymes that break down the neurotransmitter. - *Example:* **Acetylcholinesterase** hydrolyzes acetylcholine. - *Example:* **Monoamine oxidase** breaks down adrenalin. ## Electrical Synapses A specialized type of synapse for rapid signal transmission where the synaptic cleft is extremely narrow (only 0.2 nm). An action potential at the presynaptic membrane can directly depolarize the postsynaptic membrane, triggering its action potential without the need for chemical messengers. ## Classification of Neurotransmitters Neurotransmitters are classified based on their effect on the postsynaptic neuron. | Type | Function | Mechanism | Examples | | :--- | :--- | :--- | :--- | | **Excitatory** | Triggers a nerve impulse in the postsynaptic neuron. | Increases membrane permeability to positive ions like $Na^+$, causing depolarization (EPSP). | **Acetylcholine** (PNS), **Epinephrine** and **Norepinephrine** (stress response, heartbeat), **Serotonin** and **Dopamine** (mood, sleep, learning) | | **Inhibitory** | Lessens the chance of a nerve impulse in the postsynaptic neuron. | Decreases membrane permeability to $Na^+$ or increases permeability to negative ions (like $Cl^-$), causing hyperpolarization and raising the threshold for firing. | **Gamma-aminobutyric acid (GABA)**, **Glycine**, **Endorphins** (peptides that decrease pain perception) | ## Possible Questions/Answers **Q: What is a synapse?** **A:** A synapse is the junction between two neurons where a nerve impulse is transmitted from the presynaptic neuron to the postsynaptic neuron. **Q: What is the role of calcium ions ($Ca^{2+}$) in synaptic transmission?** **A:** When an action potential reaches the axon terminal, it triggers the opening of calcium channels. The influx of $Ca^{2+}$ into the terminal causes synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitters into the synaptic cleft. **Q: What is the difference between an excitatory and an inhibitory neurotransmitter?** **A:** An *excitatory* neurotransmitter (like acetylcholine) causes depolarization of the postsynaptic membrane, making an action potential more likely. An *inhibitory* neurotransmitter (like GABA) causes hyperpolarization or stabilization of the membrane potential, making an action potential less likely.